(Merge 3.4) asyncio: Move loop attribute to _FlowControlMixin

Move the _loop attribute from the constructor of _SelectorTransport,
_ProactorBasePipeTransport and _UnixWritePipeTransport classes to the
constructor of the _FlowControlMixin class.

Add also an assertion to explicit that the parent class must ensure that the
loop is defined (not None)

Lib/asyncio/proactor_events.py

@@ -21,9 +21,8 @@ class _ProactorBasePipeTransport(transports._FlowControlMixin,
 
     def __init__(self, loop, sock, protocol, waiter=None,
                  extra=None, server=None):
-        super().__init__(extra)
+        super().__init__(extra, loop)
         self._set_extra(sock)
-        self._loop = loop
         self._sock = sock
         self._protocol = protocol
         self._server = server

Lib/asyncio/selector_events.py

@@ -447,7 +447,7 @@ class _SelectorTransport(transports._FlowControlMixin,
     _buffer_factory = bytearray  # Constructs initial value for self._buffer.
 
     def __init__(self, loop, sock, protocol, extra, server=None):
-        super().__init__(extra)
+        super().__init__(extra, loop)
         self._extra['socket'] = sock
         self._extra['sockname'] = sock.getsockname()
         if 'peername' not in self._extra:
@@ -455,7 +455,6 @@ class _SelectorTransport(transports._FlowControlMixin,
                 self._extra['peername'] = sock.getpeername()
             except socket.error:
                 self._extra['peername'] = None
-        self._loop = loop
         self._sock = sock
         self._sock_fd = sock.fileno()
         self._protocol = protocol

Lib/asyncio/transports.py

@@ -238,8 +238,10 @@ class _FlowControlMixin(Transport):
     resume_writing() may be called.
     """
 
-    def __init__(self, extra=None):
+    def __init__(self, extra=None, loop=None):
         super().__init__(extra)
+        assert loop is not None
+        self._loop = loop
         self._protocol_paused = False
         self._set_write_buffer_limits()
 

Lib/asyncio/unix_events.py

@@ -369,9 +369,8 @@ class _UnixWritePipeTransport(transports._FlowControlMixin,
                               transports.WriteTransport):
 
     def __init__(self, loop, pipe, protocol, waiter=None, extra=None):
-        super().__init__(extra)
+        super().__init__(extra, loop)
         self._extra['pipe'] = pipe
-        self._loop = loop
         self._pipe = pipe
         self._fileno = pipe.fileno()
         mode = os.fstat(self._fileno).st_mode

Lib/test/test_asyncio/test_transports.py

@@ -69,7 +69,8 @@ class TransportTests(unittest.TestCase):
             def get_write_buffer_size(self):
                 return 512
 
-        transport = MyTransport()
+        loop = mock.Mock()
+        transport = MyTransport(loop=loop)
         transport._protocol = mock.Mock()
 
         self.assertFalse(transport._protocol_paused)